1. Field of the Invention
The invention relates to a method for a continuous dry milling operation of a vertical grinding mill which comprises                a vertical, closed milling container,        a screw conveyor which is arranged centrally in the milling container, the screw conveyor comprising                    a drive shaft with a central axis and            at least one screw flight which is arranged on the drive shaft, extends along a height hs up to an upper end and covers the cross-section of the milling container only partially;                        a package of grinding bodies, the package having an upper surface;        a grinding stock inlet which projects into the milling container above the package of grinding bodies,        a gas inlet which projects into the milling container for introducing gas,        a grinding stock outlet which projects out of the milling container and has a lower edge and a height h13 for discharging grinding stock and gas and        a motor for driving the screw conveyor in a direction of rotation where the at least one screw flight conveys grinding bodies upwards.        
The invention further prelates to a vertical grinding mill comprising                a vertical, closed milling container,        a screw conveyor which is arranged centrally in the milling container, the screw conveyor comprising                    a drive shaft with a central axis and            at least one screw flight which is arranged on the drive shaft, extends along a height hs up to an upper end and covers the cross-section of the milling container only partially;                        a package of grinding bodies, the package having an upper surface;        a grinding stock inlet which projects into the milling container above the package of grinding bodies,        a gas inlet which projects into the milling container for introducing gas,        a grinding stock outlet which projects out of the milling container and has a lower edge and a height h13 for discharging grinding stock and gas and        a motor for driving the screw conveyor in a direction of rotation where the at least one screw flight conveys grinding bodies upwards.        
2. Background Art
A vertical grinding mill of the generic type is known from U.S. Pat. No. 4,754,934. In this embodiment, the gas is introduced at the bottom of the milling container and flows through the package of grinding bodies and grinding stock. In the upper region of the milling container, well above the grinding stock inlet, a centrifuge is arranged on the drive shaft which causes grinding stock particles transported upwards by the gas flow to be flung away in such a way that they are immediately returned to the milling process due to the gravitational force. The gas needs to have a considerable amount of pressure, allowing the gas flow introduced into the package of grinding bodies from below to loosen up the package and to move the grinding stock particles upwards for them to be discharged at the upper end of the mill. When the package of grinding bodies and circulating grinding stock is loosened as mentioned above, the grinding effect, in other words the milling performance, is reduced. In order to keep the pressure loss in the package of grinding bodies and grinding stock within reasonable limits, the package needs to be relatively open porous, in other words there is a lower limit in terms of the size of the grinding bodies. Furthermore, the grinding stock needs to be relatively coarse. This in turn results in that the gaps between the individual grinding bodies are not sufficiently filled with grinding stock. Moreover, the energy consumption of the pressure blower is very high, the energy consumption being in the same order of magnitude as the energy consumption of the drive motor for the actual milling process.
A vertical grinding mill is known from DE 42 02 101 A1 where the grinding stock is fed into the milling container from above and discharged through a screen in the region of the bottom. In order to prevent the screen from becoming clogged or blocked, a fluid—for instance in the form of air—is introduced in the region of the bottom. A comparable vertical grinding mill is known from JP 2003 181 316 A1. The screen holes or screen slots located in the region of the bottom may become clogged by worn-out or broken grinding bodies. This in turn results in increased wear which may even cause damage to the lower ends of the screw flights. Another disadvantage is that free-flowing grinding stock such as dry silica sand flows through the package of grinding bodies at very high speeds and is therefore not subjected to a controlled milling process.
In order to avoid the aforementioned disadvantages, it is known from JP 2005 246 204 A to discharge the entire package of grinding bodies and the milled grinding stock from the milling container via a screw conveyor arranged in the bottom region. In this known embodiment, the mixture of grinding bodies and grinding stock needs to be separated outside the milling container, for instance by sieving. The grinding bodies need to be recirculated together with the new grinding stock. This requires a considerable amount of technical effort.
Furthermore, it is known from DD 268 892 A1 to blow the grinding stock out at the upper end of the vertical grinding mill by means of compressed air introduced in the bottom region or to discharge said grinding stock at the upper end of the open milling container via a circular, plane overflow edge. The disadvantage thereof is that no compact package of grinding bodies with a direct contact between grinding stock and grinding bodies is formed during operation as the grinding bodies float in the dry grinding stock. Furthermore, it may occur that grinding bodies are discharged via the overflow edge.